The present invention relates to the transmission of heterogeneous flows on time division multiplex links. It applies in particular to the transmission of an ATM flow accompanied by one or more other data flows by way of transmission interfaces of the plesiochronous hierarchy of type DS1 or E1.
DS1 or E1 links group together 64-kbit/s elementary channels (DS0) by a principle of time division multiplexed (TDM) access. According to this principle, the transmission time on the link is divided into frames of 125 μs, themselves subdivided into K timeslots (TS) each assigned to the transmission of a data byte of a respective DS0 channel. In North America (DS1 links), the number K is equal to 24, so that the overall bit rate of the link is 1.544 Mbit/s. In Europe, the links are of E1 type at 2.048 Mbit/s with K=32 (in fact, only 30 of is 32 timeslots are usable for transferring useful traffic).
A TDM link can be used to transport ATM (“Asynchronous Transfer Mode”) traffic. In ATM transmission, the data are chopped into packets of 53 bytes called cells. Each cell comprises a 5-byte header and 48 data bytes. The fifth byte of the header contains a redundancy check checksum (CRC) intended for increasing the reliability of transmission of the information of the header. The presence of this CRC byte further makes it possible, in a known manner, to detect the boundaries of the cells in the digital signal which carries them.
The way in which ATM cells are inserted on a TDM link is specified in recommendation AF-PHY-0130.00, “ATM on Fractional E1/T1”, published in October 1999 by the Technical Committee of the ATM Forum. This insertion exhibits the following characteristics:                the alignment of the bytes of the ATM cells complies with the alignment of the bytes on the E1 or DS1 link, i.e. the bytes of the ATM cells are found directly in the TSs of the frame;        the TSs carrying the bytes of the ATM cells may or may not be consecutive on the E1 or DS1 link;        the alignment of the cells is independent of the alignment of the K×64 kbit/s frames repeated every 125 μs.        
The present invention envisages in particular network deployment scenarios in which ATM virtual circuits are borne on existing TDM links carrying traffic of another kind. Such a scenario is encountered during the migration of existing infrastructures to networks using ATM technology.
In particular, the third-generation cellular radio networks of the UMTS type (“Universal Mobile Telecommunication System”) use ATM connections to transfer the data from and to the base stations deployed over the territory of coverage. These third-generation networks are set up on the basis of the existing infrastructure of second-generation networks of the GSM type (“Global System for Mobile communications”) or the like, which use TDM links with the base stations. During the setting up of the third-generation service, these TDM links may be used also to transport the ATM flows intended for the new base stations.
In such a scenario, arrangements have to be made for separating the flow of ATM cells from the other flows carried by the TDM link. In general, this requires configuration of the various items of equipment involved, so that they know from which timeslots of the TDM frame they will have to extract the relevant information. Thus, the setting up of the item of equipment for which the ATM flow is intended requires specific configuring of its TDM interface so that it can receive and synchronize itself with the ATM cells.
It may be desirable to do away with these configuring operations (“plug & play” concept). This is especially beneficial in the aforesaid example of the deployment of a new cellular radio communication network, in which a fairly large number of new base stations have to be installed at various locations.
An object of the present invention is to fulfill the above need.